The National Electrical Code (NEC) defines the requirements for the design and construction of electrical devices (e.g., light fixtures) intended for use in “hazardous locations” or “hazardous atmospheres”—terms defined in the NEC. Briefly, hazardous locations or hazardous atmospheres are defined by the conditions or substances present in the area, such as gas or vapor, dust or flyings (e.g., sawdust).
It is desirable to provide commercial light fixtures which use high intensity (e.g., halogen) light sources because of the greater amount of light they deliver efficiently. However, halogen lamps are known to operate at higher temperatures than, for example, conventional incandescent lamps. Halogen lamps provide much more light per unit of energy and typically have a much longer useful life than conventional light sources.
Typical light fixtures employing halogen lamps available at retail do not comply with the requirements of Class I, Division 1, Group C for devices operating in hazardous atmospheres. Any light fixture or lighting system which does not comply with the requirements of Class I, Division 1, Group C of the NEC may not be used in hazardous atmospheres.
In order to comply with the requirements of the NEC for operation in hazardous atmospheres, lighting systems must have an enclosure which meets certain requirements for thermal conductivity and strength—in general, the enclosure must be strong enough to contain an explosion within the device. That is, the walls must be thick enough to withstand the internal strain in the event of an internal (i.e., within the light fixture) explosion. Further, provisions must be made in the event the gas invades the enclosure and ignites within the fixture, that the flame does not escape from the fixture and ignite combustible gases in the ambient atmosphere. Moreover, the lighting system as a whole (that is, not necessarily the light source standing alone) must function at a temperature well below the ignition temperature of the surrounding atmosphere. This requires provisions for efficient transfer of considerable heat to the surrounding atmosphere.
A lighting system must also provide a means for any burning gases (in the event they should exist within the lamp) to escape from the lamp assembly because they may expand after ignition and thus create a risk of external explosion. However, the gases may be introduced to the ambient atmosphere only after they have been cooled and any possible flames “quenched”. This escape route for exploding gases is typically provided through a “flame path”. One type of qualifying quenching flame path, though not necessarily the only type of flame path, is known as the “ground surface” (i.e., machined surface) flame path. In this type of escape structure, two opposing surfaces (typically metal) are finished or ground and mated in facing configuration. The gap of the flame path must be uniform and within prescribed tolerances; and the length of the flame path and separation of the ground surfaces are also defined to insure flame quenching. This type of flame path permits gases to escape from the enclosure, but only after they have been sufficiently cooled following combustion so that they do not ignite any volatile gases in the surrounding atmosphere. Another type of quenching flame path is known as the threaded joint flame path, and this simply uses a threaded connection which permits expanding gases to escape around threaded shafts and the like to quench flame. In summary, finishing tolerances, length and separation are specified to insure that any burning gases are quenched before they are admitted into the surrounding atmosphere to avoid igniting any combustible gasses in the surrounding atmosphere.
One difficulty in using halogen lamps as the light source in a light fixture of this type is that some provision must be made for replacing the halogen lamp. In a light fixture designed for use in a hazardous location, many of the design considerations which might facilitate relamping the device, mitigate against the safety requirements for use in hazardous locations. In other words, if one had to dismantle the light fixture entirely, for example, by removing the lens from the lamp assembly in order to replace the lamp, it would be necessary to disassemble the lens from the lamp assembly. Because of the requirements of light fixtures designed for use in hazardous locations, such a design necessarily provides disassembly difficulties and increases the time and requirements for relamping. For example, as will be apparent from the embodiment of the invention disclosed herein, there may be twelve separate bolts which secure the lens to the lamp housing, and these bolts would have to be removed and replaced in accordance with prescribed procedures in order to change the lamp.